Technical Field
The present invention relates to an exhaust gas clean-up system equipped with a power generating function which cleans up exhaust gas while generating power using exhaust gas.
Background Art
From a recent view point of environmental conservation, in order to reduce harmful substances in exhaust gas discharged from heat engines including internal combustion engines such as those mounted on vehicles and external combustion engines such as those in power generation stations, exhaust gas clean-up systems are known which clean up exhaust gas with an exhaust gas clean-up device such as a catalytic converter arranged in an exhaust gas passage.
For example, exhaust gas from engines such as those mounted on vehicles which burn petroleum fuel such as gasoline or light fuel oil contains, as major harmful substances, carbon monoxide (CO), carbon hydride (HC) and nitrogen oxide (NOx). To cope with them, a three-way catalytic converter comprising precious metals, specifically platinum-group elements such as platinum (Pt), palladium (Pd) and rhodium (Rh) carried on a catalyst support made of heat-resistant oxides, mainly of silica (SiO2), alumina (Al2O3) and ceria (CeO2) is arranged in an exhaust gas passage of the engine. The three-way catalytic converter cleans up exhaust gas by converting the aforementioned harmful substances into harmless substances such as water (H2O), carbon dioxide (CO2) and nitrogen (N2) by oxidation or reduction.
In lean-burn gasoline engines and diesel engines in which a relatively small amount of fuel is burnt at high temperature and high pressure, the ratio of NOx emissions to HC and CO emissions is greater. In such engines, a device such as an NOx absorption reduction catalytic converter carrying alkaline substances capable of absorbing NOx, wherein NOx is absorbed and then reduced by creating a reduction atmosphere by, for example temporarily increasing fuel injection, or a so-called SCR (Selective Catalytic Reduction) converter, which reduces absorbed NOx using ammonia (NH3) produced by hydrolysis caused by adding a reductant such as urea water to exhaust gas, is provided in the exhaust gas passage (see Patent Document 1).
Recently, in order to curb emissions of carbon dioxide, hydrogen is attracting attention as an energy source. As a device using hydrogen as an energy source, an SOFC (Solid Oxide Fuel Cell) is known, for example. In order to enable the SOFC to operate efficiently, development of hydrogen generators for supplying hydrogen to the SOFC is being promoted. For example, a fuel cell system is disclosed in which fuel vapor generated in an evaporator is supplied to a hydrogen generator and hydrogen generated in the hydrogen generator is supplied to a fuel cell to generate power (see Patent Document 2).